This application relates to a new form of zeolite beta and to its use as a catalyst in the alkylation of aromatic compounds. More particularly, this application relates to a zeolite beta which shows substantially greater selectivity when used in the alkylation of benzene by ethylene. It is contemplated that the catalyst of this invention will be particularly valuable in production of high purity ethylbenzene, in minimizing formation of the diphenylethane which accompanies benzene alkylation by ethylene and in maximizing benzene utilization.
Ethylbenzene is the major article of commerce which is commonly made by the alkylation of benzene with ethylene. As is usual, several byproducts accompany ethylbenzene formation; a simplified summary of alkylation processes and products commonly occurring are given below. ##STR1## Zeolite beta has been found to be an effective catalyst and has gained a prominent role in the alkylation of benzene by ethylene. Although the formation of isomeric diethylbenzenes and triethylbenzenes might, at first glance, be viewed as byproducts representing a loss of ethylene, hence a reduction in efficiency of ethylene utilization, in fact each can be readily transalkylated to afford ethylbenzene as the sole alkylated benzene. ##STR2## In contrast, diphenylethane can not be converted to ethylbenzene via alkylation and thus represents a loss of ethylene and a reduction in ethylene utilization efficiency. In fact, the coproduction of diphenylethane and polyalkylated benzenes, where the latter are collectively known as heavies, represents virtually all of the reduction in ethylene utilization.
Where Y zeolite is used as a catalyst in the reaction of ethylene and benzene approximately 0.65% DPE and about 0.55 weight percent of heavies are formed, resulting in a total loss of about 1.2%. Where zeolite beta is used only about 0.4% DPE and about 0.1% heavies are formed, resulting in a loss of 0.5%. Although this improvement is small it also is very significant, resulting in zeolite beta gaining favor as a catalyst of choice for ethylbenzene production. However, formation of even the latter small amount of DPE and heavies is vexing and gave impetus to further research whose goal was to reduce losses still further.
Applicant has found that by reducing the number of active sites (on zeolite beta) which catalyze hydride transfer reaction, one obtains a product (from the alkylation of benzene) with a significant reduction in DPE content. One can modify or reduce the number of hydride transfer active ways in a number of ways such as subjecting the zeolite beta to a carbon burn or calcining the zeolite beta for an extended time in a steam atmosphere at temperatures greater than 675.degree. C.
U.S. Pat. No. 4,876,408 discloses the use of a carbon burn to modify zeolites, including zeolite beta, in order to increase it selectively for monoalkylation by at least 1.0 percentage point. In contrast to the '408 reference, the site-modified zeolite beta of this invention decreases monoalkylation selectivity, a result which is certainly unexpected in view of the contrary prior art teaching! It also is not obvious that a decrease in monoalkylation would be desirable, or even tolerable.
U.S. Pat. No. 5,227,558 discloses a steam modified zeolite beta for aromatic alkylation. However, the '558 patent discloses that the steaming dealuminates the zeolite to give a SiO.sub.2 /Al.sub.2 O.sub.3 ratio between about 50 and 350. In contrast, applicant's zeolite beta has a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 30 or less.